1. Field of the Invention
This invention relates to energy-saving electronic strobe flash apparatus. More particularly, this invention relates to energy-saving electronic strobe flash apparatus which fires dual flashtubes alternately and repeatedly to produce a composite light pulse.
2. Description of the Prior Art
Electronic strobe flash apparatus includes a chargeable capacitor that supplies a discharge current to a flashtube to produce a high intensity light pulse which lasts no longer than approximately one millisecond. Electronic strobe flash apparatus can include circuitry for quenching the flashtube to regulate its light output. In one arrangement, commonly referred to as "shunt quench," the circuitry causes the current from the firing capacitor to by-pass the flashtube to terminate its light output. Shunt-quench has a disadvantage because the energy stored on the capacitor that produces the current which bypasses the flashtube is wasted.
In a second arrangement, commonly referred to as "series-quench," the circuitry causes a switch in series with the flashtube to open to terminate the current from the capacitor, thereby quenching the flashtube. However, series-quench generally requires a relatively complicated commutating circuit to open the switch rapidly. Furthermore, the switch must be capable of handling a large current and voltage.
U.S. Pat. No. Re. 28,025 and U.S. Pat. No. 3,809,951 describe series-quench electronic strobe flash apparatus that use a commutating circuit to quench the flashtube.
However, it is desirable and expedient to use series-quench electronic strobe flash apparatus, particularly if the flash apparatus is used with an electronically-controlled camera that loads its battery heavily, and if the flash apparatus does not employ complicated commutating circuitry. U.S. Pat. No. 3,438,766 describes series-quench electronic strobe flash apparatus having dual flashtubes both of which are used as a light source and as a current-interrupting switch. A first firing circuit includes a large storage capacitor, one of the flashtubes, and a small receiver capacitor. A second firing circuit includes the receiver capacitor and the second flashtube. When the first flashtube is triggered, the storage capacitor discharges to fire the first flashtube and to charge the receiver capacitor. When the voltage on the receiver capacitor is approximately the voltage on the storage capacitor, the discharge current is interrupted, thereby quenching the first flashtube. When the second flashtube is triggered, the receiver capacitor discharges to fire it. The flashtubes are triggered alternately and repeatedly to produce a train of low intensity light pulses. After a predetermined time relating to the illumination requirements of a scene being photographed, the triggering of the flashtubes is terminated.
The flash apparatus described in U.S. Pat. No. 3,438,766 patent requires an external pulse generator for triggering the flashtubes. The generator is relatively complicated in its operation and construction, and would add significantly to the overall cost of the flash apparatus. Furthermore, the generator operates so that adjacent light pulses are separated in time such that the pulse duty factor of the pulse train is low. Flash illumination of this configuration is not ideally suitable to an application that requires exposing a fast-action scene.
U.S. Pat. No. 3,783,336 describes series-quench electronic strobe flash apparatus having a single flashtube that is supplied with firing current alternately from a pair of capacitors to produce flash illumination. A first firing circuit consists of a large storage capacitor, a first SCR, a receiver capacitor, and the flashtube. A second firing circuit consists of the receiver capacitor, the flashtube, and a second SCR. When a trigger circuit triggers the flashtube, the first SCR conducts, which causes the storage capacitor to discharge, thereby firing the flashtube and charging the receiver capacitor. When the receiver capacitor is charged to a given voltage, the first SCR turns OFF to interrupt current from the storage capacitor to the flashtube. In synchronism with the interruption of the storage capacitor current, the second SCR conducts, which causes the receiver capacitor to discharge to fire the flashtube. When the receiver capacitor is substantially discharged, the second SCR turns OFF to interrupt current from the receiver capacitor to the flashtube. In synchronism with the interruption of the receiver capacitor current, the first SCR conducts and the cycle repeats.
Because the flashtube is triggered only once for each exposure, a relatively complicated pulse control switching current is required for synchronously and repeatedly switching the 2 SCR's into conduction so that the flashtube gas is not deionized prematurely. Furthermore, the SCR's must be capable of handling high current, and they dissipate power which might otherwise be delivered to the flashtube to produce additional light.